The optical resolution of DL-.alpha.-phenylglycine has been known for many years as the commercial practice to form diastereomers with d-camphor-sulfonic acid. Also, the preferential crystallization is known to form ammonium salts (Japanese Patent Publication No. 45,388/74) or monoethylamine salts (Japanese Patent Laid-Open Publication No. 29,715/73) of N-acetyl-.alpha.-phenylglycine. These prior art methods, however, have serious drawbacks in that the former method employs d-camphor which is an expensive natural substance, while the latter method takes a complicated pathway, that is, an .alpha.-phenylglycine racemate is converted into an acetyl derivative thereof which is resolved into two isomers, each of which must, in turn, be subjected to hydrolysis of the acetyl groups.
In addition, the optical resolution of salts obtained by some amino acid with aromatic sulfonic acid derivatives have been proposed. In the case of DL-.alpha.-phenylglycine, however, its sulfonates are generally only slightly soluble in water and cannot be applied to the optical resolution. Recently, the optical resolution for the benzenesulfonate of DL-.alpha.-phenylglycine has been proposed only in an aqueous benzenesulfonic acid solution. (Ger. offen 23/9493/74)